1. Field of the Invention
The invention relates generally to roller cone drill bits for drilling earth formations, and more specifically to the geometry of cutting elements on roller cone drill bits.
2. Background Art
FIG. 1 shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig 100 used to turn a drill string 120 which extends downward into a well bore 140. Connected to the end of the drill string 120 is roller cone-type drill bit 200.
In roller cone bits, the cutting elements drill the earth formation by a combination of compressive fracturing and shearing action. Prior art milled tooth bits typically have teeth formed from steel or other easily machinable high-strength material, to which a hardface overlay such as tungsten carbide or other wear resistant material is often applied. The hardfacing is applied by any one of a number of well known methods. There are a number of references which describe specialized exterior surface shapes for the substrate.
The specialized shapes are intended to provide a cutting structure which includes more thickness of hardface overlay in wear-prone areas, so that the useful life of the teeth can be increased. Examples of such specialized substrate shapes are shown in U.S. Pat. Nos. 5,791,423, 5,351,771, 5,351,769, and 5,152,194, for example. These references show that the teeth have substantially regular trapezoidal exterior hardface surfaces. The irregular shape of the substrate outer surface is selected to provide additional hardface in the wear prone areas while maintaining a conventional exterior tooth surface.
U.S. Pat. No. 6,029,759 issued to Sue et al shows a milled tooth drill bit having teeth in a gage row (the outermost row of teeth on any cone used to maintain full drilling diameter), wherein the teeth have a particular outer surface. See for example FIG. 12B in Sue et al '759. The particular outer surface of these teeth is intended to make it easier to apply hardfacing in two layers, using two different materials. The purpose of such tooth structures is to have selected hardfacing materials positioned to correspond to the level of expected wear on the various positions about the outer surface of the tooth.
Polycrystalline diamond (“PCD”) enhanced inserts and tungsten carbide (“WC—Co”) inserts are two commonly used inserts for roller cone rock bits and hammer bits. A roller cone rock bit typically includes a bit body adapted to be coupled to a rotatable drill string and include at least one “cone” that is rotatably mounted to the bit body. The cone typically has a plurality of inserts pressed into it. The inserts contact with the formation during drilling.
The PCD layer on PCD enhanced inserts is extremely hard. As a result, the PCD layer has excellent wear resistance properties. While the actual hardness of the PCD layer varies for the inserts used in particular bit types, each type of PCD has a common failure mode of chipping and spalling due to cyclical impact loading on the inserts during drilling. Conversely, the softer, tougher tungsten carbide inserts tend to fail by excessive wear and not by chipping and spalling. Therefore, a need exists for inserts for roller cone bits that are optimized for resisting both wear and impact as encountered during drilling.